Enhanced diffusion welding

ABSTRACT

Surfaces of unrecrystallized alloys are sanded and polished. This is followed by a two-step welding process wherein the strength of the parent metal is retained at the weld joint. The first step forces the surfaces into intimate contact at a temperature where the metal still has good ductility. The second step causes diffusion, rerystallization, and grain growth across the original weld interface.

United States Patent [191 Holko et al.

[451 Sept. 11, 1973 ENHANCED DIFFUSION WELDING Inventors: Kenneth H.llolko, Strongsville;

Thomas J. Moore, Berea, both of Ohio Assignee: The United States ofAmerica as represented by the Administrator of the National Aeronauticsand Space Administration, Washington, DC.

Filed: Sept. 14, 1972 Appl. No.: 289,033

US. Cl 219/91, 29/497, 219/117 Int. Cl B23k 19/00 Field of Search219/91, 96, 117,

References Cited UNITED STATES PATENTS 8/1972 Blum et a1. 29/497 OTHERPUBLICATIONS J. M. Parks, Recrystallization Welding, Welding Journal, U.32(5), 2095-2225, (1953).

Anderson 219/91 Primary Examiner-J. V. Truhe Assistant ExaminerB. A.Reynolds AttorneyN. T. Musial et al.

[57] ABSTRACT 10 Claims, No Drawings ENHANCED DIFFUSION WELDINGBACKGROUND OF THE INVENTION The invention described herein was made byemployees of the United States Government and may be manufactured andused by or for the Government for governmental purposes without thepayment of any royalties thereon or therefor.

PRIOR ART This invention is concerned with using solid state welding toachieve strong welds with metals that undergo recrystallization uponheating. The invention is particularly directed to weldingdispersionstrengthened nickel alloys.

Conventional brazing and fusion welding methods, such as resistance spotwelding, have been used to join certain alloys. Fusion welding andbrazing methods result in weldments having approximately 50 percent ofthe parent metal strength.

Brazing dispersion-strengthened nickel alloys, such as Ni-ThO,, NiCr-Th,NiMoThO,, and Ni- -CrAl--Th0,, involves degradation of parent metalstrength. Diffusion occurring between the braze alloy and parent metalat elevated temperatures causes porosity to form in the parent metal aswell as in the braze alloy. The thoria dispersion, which is criticaltothc development of strength, may be destroyed. Also the textureproduced by thermomechanical processing may be lost. In addition, thebraze alloy is not as strong at elevated temperatures as thedispersion-strengthened nickel alloy. All of these factors cancontribute to weak brazements.

Fusion welding necessarily involves melting of thedispersion-strengthened nickel alloy, and the thoiriadispersion is lost.Thus the strengthening effect of the thoria is lost, and the weldment isrelatively weak.

Solid-state welding using both direct resistance heating at the jointand indirect heating from resistance heated elements has been proposedfor joining alloys.

Solid-state welding is desirable because melting is avoided and'foreignmaterial need not be introduced at the joint. However, conventionalsolid-state welds have proven to be weak and brittle when tested atelevated temperatures. A thin recrystallized band of small grains formsat a continuous weld line which acts as a boundary. The continuous weldline and the small grains cause weakness at elevated temperatures. Thisresults in joint failure at low stresses. Typically, joint efficiency is0 to 60 percent. Also, unwelded areas occur sporadically at the weldline.

SUMMARY OF THE INVENTION These problems have been solved by enhanceddiffusion welding in accordance with the present invention whereinuncrystallized nickel alloys are used as a starting material. Sandingand electropolishing provide very flat surfaces free of stored energywhich prevents the formation of small grains at the interface.Recry'stallir zation produced during the weld cycle eliminates the weldline.

A dispersion-strengthened nickel alloy is first heated to l,300F at30,000 psi for 1 hour. This is followed by heating to 2,175'F at 2000psi for 2 hours. Recrystallization occurs upon heating, and the originalweld interface is removed by grain growth across it.

OBJECTS OF THE INVENTION It is, therefore, an object of the presentinvention to provide a method of solid-state weldingdispersionstrengthened materials without losing strength in the parentmetal or at the weld.

Another object of the invention is to provide a method of welding thedispersion-strengthened nickel alloys Ni-ThO,, NiCr--ThO NiMoThO andNi-Cr-AlThO A further object of the invention is to provide a method ofsolid state welding unrecrystallized alloys without producing a weldline at the interface.

These and other objects of the invention will be apparent from thespecification which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT Sheets of a nickel base alloy,commercially designated as TDNiCr, were welded in accordance with thepresent invention to illustrate the advantages of enhanced diffusionwelding. Normally commercial TD-- NiCr is recrystallized, but speciallyprocessed sheets of this material were prepared. The special processingconsisted of omitting the final recrystallization heat treatment that isnormally given to commercial TD- NiCr after thennomechanical processing.Thus the specially processed TD-NiCr was in the unrecrystallizedcondition in which the grain size is extremely fine. The sheets had anominal thickness of 1.6 millimeters.

TD-NiCr has a nominal composition of 20 percent chromium, 2 percentthoria and the remainder nickel. The alloy has good high temperaturestrengthand oxidation resistance. The TD-NiCr derives its hightemperature. and oxidation resistance. The TD--NiCr derives its hightemperature strengthfrom mechanical working of the Ni-20 weight percentCr matrix which contains a fine dispersion of Th0 particles. Use of thismetal has been suggested for applications where metal temperatures mayreach about 2,200F in an oxidizing environment. These metals havepotential applications in jet engine components and heat shield panelsfor space shuttle vehicles. Y

Commercial TD-NiCr sheets having the same thickness were also welded forcomparison purposes. Commercial TD-NiCr is made from the specialprocessed material by recrystallization at 2,150F for 2 hours. This heattreatment is termed as stress relieving. The commercial material is moreductile at room temperature than the special processed material, and itis more formable.

, condition had IZO-grit sanded surfaces with the surface scratchesparallel to the principal rolling direction. The surfaces were preparedfor welding in three different ways. Some weld specimens were preparedby cleaning the as-received IZO-grit sanded surfaces with methanol andtrichlorotrifluorethane before welding. Other specimens were prepared bysanding both sides with 180- grit paper, sanding the side to be weldedwith 600-grit paper, cleaning with methanol and trichlorotrifluorethane,and storing in trichlorotrifluorethane to minimize oxidation. Finally,some specimens were sanded to 600-grit paper as described, thenelectropolished, cleaned with methanol and trichlorotrifluorethane, andstored in trichlorotrifluorethane. The specimen surfaces in contact withthe molybdenum rams were coated with A1 to prevent sticking.

After overlapping the specimens approximately onehalf inch, a vacuum of2X10 torr was attained in the weld chamber. The specimens were heated tothe welding temperature, the welding force was applied, and diffusionwelding was achieved. No measurable deformation was recorded afterwelding.

A two-step weld cycle shown in TABLE I produced the best welds. Thiscycle consisted of a low temperature, high pressure portion whereinthematerial was heated to about 1,300F at a pressure of 30,000 psi for 1hour. This was followed by a high temperature, low

pressure portion wherein the niaterial is heated to about 2,175F and2,000 psi for 2 hours. Both steps were performed in a vacuum of 2 l0torr.

TABLE I. PREFERRED WELD CYCLE Pressure Other welds were made using thecycles shown in TABLE 11 in a vacuum of 27(10 torr. The welds producedwith these cycles were not as good as those using the two step weldcycle shown in TABLE 1.

The weldments were evaluated both metallographically and by tensile andcreep-rupture shear tests. The same specimen configuration was machinedfrom the weldments for tensile-shear and creep-rupture shear testing.This test specimen configuration had a test section with a 1.6millimeter overlap and a 4.3 millimeter width. All specimens were heattreated at 2,300F for l hour'in vacuum before testing to stress relievethe weldment. Specimens were tensile shear tested in air at 2,000F at acrosshead speed of 1.3 millimeters per minute. Creep-rupture shear testswere conducted at 2,000F in air with deadweight loading of about 16pounds.

In order to compare the weld strengths to parent metal strength, parentmetal shear specimens were machined and tested. Testing conditions werethe same as for the weld specimens.

TABLE ll OTHER WELD CYCLES Commercial TDNiCr l20-grit sanded 1900 6,0000.5. 1.0 2000 9,000-l5.000 0.05

IZO-grit sanded l H VA '7 W plus electropolished 2000 10.000 0.05

600-grit sanded 1750-2000 9.500 0.05 1900 6,000 0.5. 1.0

GOO-grit sanded 2000 10,000 0.05 1900 6,000 1.0 1500 14,000 3.0 21003,000 1.0

The 2,000F tensile-shear strength data for the parent materials is shownin TABLE III. The material that was specially processed plus heattreated at 2,300F for 1 hour exhibited strengths equivalent to that ofthe heat-treated commercial TDNiCr. The average parent metaltensile-shear strength is 9,500 psi at 2,000F.

The welds made in the specially processed TDNiCr had an averagetensile-shear strength of about 7,500 pounds per square inch. Thisrepresents a joint efficiency of 79 percent. However, the speciallyprocessed TDNiCr weldments failed mostly in the parent material, awayfrom the original weld interface. Close inspection of the planar view ofthe failed specimen showed a tension type of fracture away from the weldin addition to the shear fracture. This is caused by the bending momenton the joint that exists when the shear load is applied because of theweld specimen configuration. This bending moment is much less whentesting the parent metal configuration. Because of the bending momentand location of the fracture it is believed the joint efficiencyactually approaches percent. Therefore, parent metal strength wasattained in tensile shear testing.

TABLE III PARENT METAL STRENGTHS AT Parent TDNiCr creep-rupture sheardata was obtained and is shown in TABLE III. This data indicates a 100hour life at about 3,100 psi at 2,000F. Comparison of parent and weldstrengths in TABLE IV shows that parent metal strength has also beenobtained in creep-rupture shear tests.

While a preferred embodiment of the invention has been described it willbe appreciated that various modifications may be made to the processwithout departing from the spirit of the invention or the scope of thesubjoined claims.

TABLE IV STRENGTHS OF WELDS AT 2000F What is claimed is: 1. A method ofsolid state welding members of uncrystallized materials comprising thesteps of providing mating surfaces on the members to be welded that areflat and free of stored energy thereby preventing the formation of smallgrains,

assembling said members with said mating surfaces in intimate contact,

heating the assembled members to a first temperature at which saidmembers are substantially ductile,

applying a first pressure to said assembled members while heated to saidfirst temperature,

heating the assembled members to a second temperature which issubstantially higher than said first temperature, and applying a secondpressure to said members which is substantially lower than said firstpressure while heated to said second temperature to cause diffusion,recrystallization, and grain growth across the original interface. 2. Amethod of solid state welding as claimed in claim 1 wherein the membersare of a material which undergoes recrystallization and grain growthupon heating.

3. A method of solid state welding as claimed in claim 2 wherein themembers are of a dispersionstrengthened alloy.

4. A method of solid state welding as claimed in claim 3 wherein themembers are of a nickel alloy.

5. A method of solid state welding as claimed in claim 4 wherein themembers are of a nickel alloy selected from a group consisting of NiTh0NiCrThO NiMo--ThO and NiCrAlThO 6. A method of solid state welding asclaimed in claim 5 wherein the members are of a dispersionstrengthenednickel alloy that is unrecrystallized prior to heating.

7. A method of solid state welding as claimed in claim 6 whereinthemembers are heated to a first temperature of about 1,300F at a firstpressure of about 30,000 psi for about 1 hour.

8. A method of solid state welding as claimed in claim 7 wherein themembers are heated to a second temperature of about 2,100F at a secondpressure of about 2,000 psi for about 2 hours.

9. A method of solid state welding as claimed in claim 1 wherein themembers are heat treated subsequent to the application of the secondpressure.

10. A method of solid state welding as claimed in claim 9 wherein themembers are heat treated at about 2,300F for about 1 hour.

2. A method of solid state welding as claimed in claim 1 wherein the members are of a material which undergoes recrystallization and grain growth upon heating.
 3. A method of solid state welding as claimed in claim 2 wherein the members are of a dispersion-strengthened alloy.
 4. A method of solid state welding as claimed in claim 3 wherein the members are of a nickel alloy.
 5. A method of solid state welding as claimed in claim 4 wherein the members are of a nickel alloy selected from a group consisting of Ni-ThO2, Ni-Cr-ThO2, Ni-Mo-ThO2, and Ni-Cr-Al-ThO2.
 6. A method of solid state welding as claimed in claim 5 wherein the members are of a dispersion-strengthened nickel alloy that is unrecrystallized prior to heating.
 7. A method of solid state welding as claimed in claim 6 wherein the members are heated to a first temperature of about 1,300*F at a first pressure of about 30,000 psi for about 1 hour.
 8. A method of solid state welding as claimed in claim 7 wherein the members are heated to a second temperature of about 2,100*F at a second pressure of about 2,000 psi for about 2 hours.
 9. A method of solid state welding as claimed in claim 1 wherein the members are heat treated subsequent to the application of the second pressure.
 10. A method of solid state welding as claimed in claim 9 wherein the members are heat treated at about 2,300*F for about 1 hour. 